Clocks have been used for centuries as tools for a wide variety of tasks. Generally, clocks rely on some type of device that oscillates at a given frequency known as a clock source or an oscillator. Oscillations from the oscillator may incrementally measure the passage of time. The most common oscillators known in the art are crystal oscillators. A crystal of a substance such as quartz may be cut in such a way that when it is excited by an electric current it oscillates at a given frequency.
In some undertakings, such as downhole seismic measurements, precise time-sensitive measurements are desirable, requiring very accurate clocks. Typically, oscillators are only accurate to a certain level, and are subject to varying degrees of error. In addition, changes in temperature and pressure also affect the frequency of an oscillator and may introduce error into clock measurements. Typically, crystal oscillators are stabilized by controlling the environment in which the crystal is placed
Another device which uses the pulses from an oscillator is a baud rate generator. In many situations dealing with network communications, a baud rate generator is required. A baud rate generator relies on regular pulses received from an oscillator in order to produce pulses at a rate at which a network device may read symbols on a line of code or write symbols to a line of code. Generally, baud rate generators are equipped with a prescaler that divides the number of pulses produced by the oscillator by some integer for every pulse emitted by the generator. However, selecting baud rates from a fixed oscillator may be difficult if only integer divisors are allowed. Often in order to change the baud rate, the oscillator must be exchanged for one that has pulses which may be divided by an integer to produce the correct rate. A baud rate generator may also be subject to the effects of oscillator drift.
Several attempts have been made in the art to adjust the frequency of clock or baud rate generator pulses. For instance, U.S. Pat No. 4,868,514 to Azevedo discloses an apparatus and method for digital correction of oscillator drift by providing phase alignment between two clock signals running at nearly the same frequency. Phase alignment is provided by fashioning a delay for one of the clock signals through selection of various lengths of a variable delay path formed from a series of logic circuits.
U.S. Pat No. 4,602,375 to Inukai discloses a method and system for onboard clock correction in a satellite by means of drift prediction. Clock drift is predicted by a polynomial function and corrected by sending a clock correction value to the satellite.
U.S. Pat No. 6,711,230 to Nicholls discloses an apparatus in which an input reference timing signal oscillator of a phase-locked loop has a computer algorithm which adaptively models the multiple frequencies of an oscillator following a training period. The oscillator is part of a phase-locked loop and the oscillation frequency thereof is controlled in response to the phase detector output.
U.S. patent application Ser. No. 10/878,147 to Hall et al. filed on Jun. 28, 2004, and herein incorporated by reference, discloses an apparatus and method for compensating for clock drift wherein a number loaded into a prescaler is adjusted according to clock drift. The apparatus is designed for use in a downhole network with an oscillator whose pulses can be divided by an integer in a prescaler in order to obtain the correct frequency of clock pulses. Using information received over the network, clock drift may be corrected using the apparatus.